Neuropathic pain represents an unmet medical need. Its impact on the quality of life in patients treated with anti-cancer agents such as oxaliplatin, or suffering from the consequences of inflammatory or mechanic lesions of nerves, is dramatic. Despite the progress in the field, available drugs have limited efficacy, for example opioids or paracetamol, and unavoidable side effects. Transmission of pain stimuli to the central nervous system involves multiple structures but key checkpoints reside in the dorsal root ganglia (DRG), which receive signals from the periphery. DRG neurons are excitable cells expressing, among others, the hyperpolarization-activated, cyclic nucleotide-gated (HCN) current named Ih, whose properties resemble the pacemaker current If typical of the cardiac sinus node: for this reason, DRG neurons are termed ‘pacemakers of pain’. Indeed, the proteins which form the channels are identical to the cardiac ones: at variance with the heart, the prevalent isoforms in DRG are HCN1 and HCN2 (Schnorr et al., 2014), not HCN4. Are these channels relevant for pain transmission? Yes and no, and this is a key point. Their role in physiological conditions is modest; however, upon pathological stimuli such as inflammation or lesions, they switch to an ‘over-functional’ state, leading to allodynia or hyperalgesia. The readers can refer to extensive reviews for details (Sartiani et al., 2017). Of course, a target which triggers inappropriate chronic pain – and not nociceptive or other sensorial stimuli – has quickly attracted the interest of pharmacologists. However, how to modulate the channels is tricky, especially because HCN channels are widely distributed in excitable cells – cardiac pacemakers, neurons in the retinal, peripheral and central nervous system – and the risk of side effects may overcome the benefit. Thus, the second bottleneck is the availability of a selective blocker of HCN channels, able to function as an anti-nociception agent in DRG neurons. The only commercially available drug is ivabradine, a compound which blocks all HCN isoforms prescribed as a bradycardic agent in cardiac diseases. Indeed, there is anecdotical evidence of ivabradine's analgesic effect against cold allodynia, but clinical trials are not conclusive and, indeed, unintended bradycardia might occur. The present work by Saponaro et al. stems from an original idea: to target an intrinsic property of the HCN2 protein, which contains a COOH-terminal domain binding cyclic nucleotides – namely cAMP (CNBD). It is important to focus on this essential point: of the two main isoforms in DRG neurons, the HCN2 is extremely sensitive to cAMP, at variance with the HCN1. Inflammatory processes increase cAMP levels in pain-sensing neurons, which in turn cause the hyperfunction of HCN2 channel proteins and trigger the acute phase of neuronal hypersensitivity. Targeting the HCN2-cAMP binding sequence might switch off the signal and stop pain transmission – this is the idea – with minor effects on other types of physiological neuronal transmission. The small protein TRIP8bnano, developed by Saponaro et al. since 2018, exploits the similarity to a beta-subunit of HCN channels and the selectivity for the CNBD of the HCN channel, while sparing other cAMP-binding proteins. Of course, translating this evidence (obtained in vitro with the TRIP8bnano protein) to animal models of neuropathic pain required a different strategy for in vivo administration. The intrathecal injection of a plasmid containing the TRIP8bnano in rat models of mechanical allodynia or thermal hyperalgesia (the most relevant proxy of neuropathic pain in humans) proved both the targeted expression in DRG neurons by histochemistry assays, and the analgesic efficacy evaluated by assessing the pain behaviour in rats. Yet, an intrathecal route of administration is far from the ideal, even in severe conditions associated with chronic pain. However, recent development of novel therapeutic strategies, e.g. for dyslipidaemias (Brandts & Ray, 2023), paves the way also for small proteins or gene expression modulation in other settings. On the other hand, the translational value of present findings by Saponaro and co-workers also resides in the evidence of an analgesic, long-lasting effect achievable by blocking the cAMP-mediated gating of HCN2. As also inferred by previous studies using small molecules (Resta et al., 2018), the elegant results shown in this paper represent a decisive point in favour of the Ih hypothesis, and specifically of the HCN2 isoform, and the cAMP-signalling pathway as checkpoints of the pain pacemaker triggers. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. None declared. Sole author. None. Open access publishing facilitated by Universita degli Studi di Firenze, as part of the Wiley - CRUI-CARE agreement.
Elisabetta Cerbai (Sun,) studied this question.